1. Field of the Invention
The present invention relates to a construction machine having a hydraulic pump control system.
2. Description of the Related Art
Generally, for preventing an engine stall in a construction machine, there is performed an engine speed sensing control (hereinafter referred to as xe2x80x9cESS controlxe2x80x9d) in which a pump horsepower (pump discharge) is controlled in accordance with number of revolutions of an engine, i.e., engine speed or engine revolutions.
According to the ESS control, when pump load (pump pressure) increases and the engine speed decreases, pump flow rate is decreased. In this case, a control is made so that the pump horsepower becomes small in reply to a large load and becomes large in reply to a small load, and therefore an engine stall is prevented.
However, the conventional pump control system involves the following problems.
When a construction machine is operated at a low temperature, for example in the winter season, the temperature of the hydraulic oil and that of the engine oil are low and highly viscous just after start-up of the engine. Under the resistance of these oils, the engine torque increases.
If in this state there is performed a work of a large load, for example if there is performed an arm pushing operation for an arm as an excavating attachment in a hydraulic excavator, there is conducted a pump horsepower control based on only engine speed by ESS control as is the case with the control at room temperature despite the engine load being large under the aforesaid oil resistance. As a result, the engine torque becomes overtorque, causing a great damage to the engine.
If the viscosity of the hydraulic oil is high, the reaction of a pump regulator which is operated with the hydraulic oil becomes dull and the response of the pump to a discharge rate command is delayed. Consequently, in a work under a greatly varying load, hunting is apt to occur in the pump discharge rate commandxe2x86x92pump discharge rate ESS control system.
It is an object of the present invention to provide a construction machine having a pump control system which can prevent the occurrence of engine overtorque and hunting at low temperatures.
The construction machine of the present invention comprises an engine; a hydraulic pump which is actuated by the engine; a hydraulic actuator circuit adapted to use the hydraulic pump as a hydraulic oil source; a pump regulator adapted to control discharge rate of hydraulic oil or working oil discharged from the hydraulic pump; an engine speed detecting means such as an engine speed sensor adapted to detect the number of revolutions of the engine; a temperature detector adapted to detect temperature of the hydraulic oil; and a control means adapted to control the discharge rate of the hydraulic pump through the pump regulator. The control means is constructed so as to perform an engine speed sensing control in which the pump flow rate is controlled in accordance with engine speed or engine revolutions. The control means is further constructed so as to perform a low-temperature horsepower control such that in a temperature region wherein the temperature of the hydraulic oil is lower than a preset temperature the pump flow rate relative to the engine speed is decreased to a lower level than when the temperature of the hydraulic oil is not lower than the preset temperature.
In this connection, when the temperature of the hydraulic oil does not reach the preset temperature (at a lower temperature than the preset temperature), it is possible to perform a low-temperature horsepower control in which the pump horsepower is set lower than when then the temperature is not lower than the preset temperature (at room temperature). With this control, the engine load is diminished, so that it is possible to prevent overtorque of the engine at a low temperature.
Moreover, according to the low-temperature horsepower control, the absolute value of the pump flow rate is low and the amount of change in the pump flow rate caused by a load variation becomes small, so that hunting is difficult to occur.
The temperature of engine oil also contributes to overtorque. However, as the temperature of the hydraulic oil rises, the temperature of the engine oil also rises, so that temperature of the engine oil can be detected indirectly by detecting the temperature of the hydraulic oil. Therefore, even without separately detecting the temperature of the engine oil, the desired object can be achieved by detecting the temperature of hydraulic oil and controlling the horsepower in the manner mentioned above.
Alternatively, the temperature of the hydraulic oil may be detected indirectly by detecting the temperature of the engine oil. Further, since the temperature of engine cooling water is correlated with the temperature of the hydraulic oil, the temperature of the hydraulic oil may be detected indirectly by detecting the temperature of the engine cooling water.